Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
For example, U.S. Pat. No. 4,054,881 describes a tracking system using three coils to generate electromagnetic fields in the vicinity of an object being tracked. The fields generated by these three coils are distinguished from one another by open loop multiplexing of time, frequency or phase. The signal currents flowing in three orthogonal sensor coils are used to determine the object's position, based on an iterative method of computation.
U.S. Pat. No. 5,391,199, filed Jul. 20, 1993, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference, describes a system for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
PCT patent publication WO/96/05768, filed Jan. 24, 1995, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference, describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates. As in the case of the '539 patent application described above, the radiator coils preferably operate simultaneously at different frequencies, for example at 1000, 2000 and 3000 Hz, respectively.
The above tracking systems rely on separation of position-responsive signals into components, most typically frequency components, wherein each such component is assumed to correspond uniquely to a single radiator coil, in a known position, radiating a magnetic field having a regular, well-defined spatial distribution. In practice, however, when a metal or other magnetically-responsive article is brought into the vicinity of the catheter or other object being tracked, the magnetic fields generated in this vicinity by the radiator coils are distorted. For example, the radiator coil's magnetic field may generate eddy currents in such an article, and the eddy currents will then cause a parasitic magnetic field to be radiated. Such parasitic fields and other types of distortion can lead to errors in determining the position of the object being tracked.